Metals such as cadmium, lead, and arsenic are highly toxic to living organisms. Wastewater discharge may be a primary source of heavy metal release into the environment. The removal of heavy metal ions from industrial wastewater has been given much attention in the last decade, because such components can accumulate in living organisms. Upon their accumulation in the human body, these toxic metals may cause kidney failure, nerve system damage, and bone damage, as well as other serious diseases. The necessity to reduce the amount of heavy metal ions from the environment has led to an increasing interest in technologies that selectively remove such toxic metals.
There are 35 metals that are of concern because of occupational or residential exposure; 23 of these are the heavy elements or “heavy metals.” Heavy metals are chemical elements with a specific gravity that is at least 5 times the specific gravity of water. Small amounts of these elements are common in our environment and diet and in some cases are actually necessary for good health, but large amounts of any of them may cause acute or chronic toxicity. Heavy metal toxicity can result in damaged or reduced mental and central nervous function, lower energy levels, and damage to blood composition, lungs, kidneys, liver, and other vital organs. Long-term exposure may result in slowly progressing physical, muscular, and neurological degenerative processes that mimic Alzheimer's disease, Parkinson's disease, muscular dystrophy, and multiple sclerosis. Allergies are not uncommon and repeated long-term contact with some metals or their compounds may even cause cancer.
A particular heavy metal of concern is iron. Iron is an essential and ubiquitous element in all forms of life involved in a multitude of biological processes and essential for many critical human biological processes. Yet, the presence of excess iron in the body may lead to toxic effects.
Iron overload is a serious complication in patients that have β-thalassemia and is the focal point of its management. In patients that do not receive transfusions, abnormal iron absorption can produce an increase in the body iron burden, which is evaluated to be in the 2-5 gram per year range. Patients that receive treatments that include regular blood transfusions can lead to double this amount of iron accumulation. Iron accumulation introduces progressive damage in liver, heart, and in the endocrine system if left untreated. The available iron is deposited in parenchymal tissues and in reticuloendothelial cells. When the iron load increases, the iron binding capacity of serum transferrin is exceeded and a non-transferrin-bound fraction of plasma iron (NTBI) appears. The NTBI can generate free hydroxyl radicals and induces dangerous tissue damage. Iron accumulates at different rates in various organs, each of which react in a characteristic way to the damage induced by NTBI and by the intracellular labile iron pool (LIP). Current treatments for iron overload diseases include chelation therapy to chelate the iron and reduce its bioavailability. In one example, chelation therapy can be performed with desferoxamine (DFO), which is administered by subcutaneous infusion. Drugs that can be administered orally include deferiprone and Exjade. DFO therapy has reportedly been associated with several drawbacks including a narrow therapeutic window and lack of oral bioavailability. As a result, it requires administration for 8-12 hours per day by infusion. DFO can not be readily absorbed through the intestine and must be injected intravenously thus, is not an ideal chelator since systemic side effects are likely. Furthermore, concerns have arisen over its use due to numerous significant drug-related toxicities. Serious adverse effects such as neutropenia, agranulocytosis, hypersensitivity reactions, and blood vessel inflammation have also been reported upon the oral application of deferiprone and Exjade.
One possible method of avoiding the use of systemic iron chelators is to inhibit iron absorption from the gastrointestinal tract by orally available, non-absorbed iron chelators that selectively sequester and remove excess dietary iron from the GI tract. Non-absorbed polymer therapies that act by sequestering a number of undesired ionic species in the gastrointestinal tract have been successful clinically. Using non-absorbed polymer therapies is particularly relevant to thalassemia intermedia and hemochromatosis. Iron binding polymers have considerable potential in this therapeutic approach as they can effectively bind iron irreversibly to form nontoxic, inert complexes that are not absorbed by the gastrointestinal tract, thereby reducing the absorption of iron from the intestine.
Microorganisms have developed a sophisticated Fe(III) acquisition and transport systems involving siderophores. Siderophores are low molecular weight chelating agents that bind Fe(III) ion with high specificity. The iron binding affinity of siderophores dramatically exceeds that of iron chelating therapeutics currently available. Enterobactin, a naturally occurring tris-catechol siderophore, is the most powerful Fe(III) chelator known with a relative iron binding constant of 35.5. Since nearly all iron is bound in vivo, next generation iron chelators must achieve significantly higher iron binding and selectivity with low toxicity and side effects. Researchers have synthesized small molecule siderophore mimetics; however, compounds that directly mimic siderophores would be expected to enhance bacterial recruitment of iron. What is needed is a novel iron chelator that binds iron tightly and removes it from the body.